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New perspectives to save cultural heritage
Altan, M. Orhan

CIPA 2003 XIX th International Symposium, 30 September - 04 October, 2003, Antalya, Turkey
Figure 3. Longitudinal apse section and one facade
of the transept. There USM on vector map and on raster image
rectification are shown
high resolution scanning subjected to reduction procedure.
Only for the transept, Cyrax 2500 laser (by Leica) has been used,
with sampling step of about 1.5 cm; in this way all the transept has
been covered. For more complex details, as for example, capital
blocks, a sampling step of 6 mm has been set.
In all, 20 range maps, with an overlapping of about 40% have
been generated. This overlap has been performed not only to align
3D images but also to cover undercuts and hidden zones. Laser
scanner has been mounted on its tripod or simply placed on the
scaffoldings, at various levels, to avoid great inclinations.
Actually, 3D modeling of real free-form surfaces consists of the
following steps: registration, pre-processing, mesh generation,
post-processing, texturing. Range map registration has been
carried out with Cyclone software (by Cyra). To obtain the
transept complete model, 20 range maps have been registered,
13 for right zone and 7 for left zone. In order to establish
connection with the two parts, (interrupted by the apse) and
in order to set a unique reference system for all type of data,
specific targets, topographically measured, have been used as
additional constrains in the range data alignment. These target
are automatically recognized by the software which calculates the
Figure 4. Laser scanner setup
Figure 5. Example of range map registration:
we can see target position, automatically determinated, used
to rototranslate the model in the topographic reference system
centre. The registrations have been performed in scan block; for
each one, 5 or 6 targets were available, 3 for the rototranslation
in the reference system and the other as control points.
All following data processing has been done on portions of the
entire model, to be able to manage acceptable file dimension and
to reduce elaboration time. Also visualisation became modifiable
in real time. At the end, all blocks have been reassembled in order
to create a unique model.
Figure 6. A view of the model, integrated, in apse area,
with the photogrammetric restitution
In the pre-processing phase we have removed unrelated elements
to the object of survey. This is an operation only partly automatic,
for instance, using on-distance based filter. To refine the selection
of these elements, manual operation is required (Figure 7).
Also, noise reduction enhances the scan quality by removing excessive
speckling with statistic methods, as we show in figure 8.
In figure 9, there is an example of clusterization of registered
block of range map. The partition has been performed marking
the boundary of significative architectonic portions of about one
million of points.
The conversion of the measured data into a consistent polygonal
surface implies the generation of triangular surfaces (FigurelO),
satisfying some quality requirements, and mesh optimization
Figure 7. Examples of unrelated elements